Thin film transistors (or TFTs) are used as switching elements for flat panel display devices such as liquid crystal display devices in which pixels are arranged in an active matrix type or an organic electric field emission display device, for example.
FIG. 1 is a sectional view showing a thin film transistor generally used in a flat panel display device, etc.
Referring to FIG. 1, a thin film transistor 6 comprises a gate electrode 8 formed on a substrate 2 and supplied with a gate signal, a source electrode 10 supplied with a data signal, a drain electrode 12 spaced from the source electrode 10 by a channel of the thin film transistor 6, an active layer 14 overlapped with the gate electrode 8 with a gate insulating film 24 therebetween for forming the channel of the thin film transistor 6 between the source electrode 10 and the drain electrode 12, and an ohmic contact layer 16 for reducing contact resistance between the active layer 14 and the source electrode 10 and the drain electrode 12, respectively.
The thin films of the TFT 6 are made of inorganic materials. The inorganic materials are disposed on the substrate 2 by a deposition technique such as, for example, sputtering or plasma enhanced chemical vapor deposition (PECVD), and patterned to form the thin films. However, since the deposition equipment tends to be expensive, the fabricating cost of the TFT 6 may be increased by using such deposition techniques to form the inorganic thin films. Moreover, since the photolithography processes for forming the gate electrode 8, the source electrode 10, the drain electrode 12, the active layer 14 and the ohmic contact layer 16 also include exposure and development processes, the fabricating cost of the TFT 6 may be further increased.
In view of the drawbacks of thin film transistors made from inorganic materials (hereinafter, referred to as “inorganic TFT”), a thin film transistor including an organic semiconductor layer (hereinafter, referred to as “organic TFT”) has been suggested.
FIG. 2 is a sectional view showing the organic TFT.
Referring to FIG. 2, an organic TFT 56 comprises a source electrode 60 formed on a substrate 52 and supplied with a data signal, a drain electrode 62 opposed to the source electrode 60 and spaced a predetermined distance from the source electrode 60, an organic semiconductor layer 64 formed over the source electrode 60 and the drain electrode 62, an organic gate insulating film 74 formed over the organic semiconductor layer 64, and a gate electrode 58 formed on the organic gate insulating film 74 and overlying the area between the source electrode 60 and the drain electrode 62.
A method of fabricating the organic TFT will be described with reference to FIG. 3A to FIG. 3D.
Referring to FIG. 3A, a source/drain metal layer is deposited on the substrate 52 by a deposition technique such as sputtering or PECVD, for example. Then, photolithography and etching processes are carried out to form the source electrode 60 and the drain electrode 62. The source/drain metal may be Au, Mo, Ti, Ta, Mo-alloy, Cu or an Al-group metal, for example.
Next, the organic semiconductor material is formed on the substrate 52 provided with the source electrode 60 and the drain electrode 62 by a method such as spin coating or spinless coating, for example, and hardened to form the organic semiconductor layer 64 shown in FIG. 3B. The organic semiconductor material may be made of, for example, a pentacene-group material, polythiophene-group material, or a polyarylamine-group material.
Next, an organic insulating material is coated on the substrate 52 provided with the organic semiconductor layer 64 by a method such as spin coating or spinless coating, for example, to form the organic gate insulating film 74 shown in FIG. 3C. The organic insulating film may be made of an acrylic organic compound such as, for example, BCB or PFCB.
After that, a gate metal layer is deposited on the gate insulating film 74 by a deposition technique such as sputtering. Then, photolithography and etching processes are carried out to form the gate electrode 58 shown in FIG. 3D. The gate metal may be Cr, Mo, Al or an Al-group metal such as AlNd, for example.
In the organic TFT 56 having the same structure as shown in FIG. 2 and formed by the method shown in FIG. 3A to FIG. 3D, the organic semiconductor layer 64 and the organic gate insulating film 74 may be formed by a relatively simple method such as spin coating or spinless coating. Accordingly, the organic TFT 56 has a simpler fabrication process than the inorganic TFT 6 with reduced usage of expensive deposition equipment. It is therefore possible to reduce the fabricating cost of the TFT.
But, the organic semiconductor layer 64 of the organic TFT 56 may be damaged by a strip liquid used for removing photo-resist to form the gate electrode 58 in a subsequent process.
Further, the organic semiconductor layer 64 of the organic TFT 56 may be deteriorated by heat in a deposition process for a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO) upon forming of a pixel electrode (not shown) in a subsequent process.
Furthermore, the organic semiconductor material for forming the organic semiconductor layer 64 of the organic TFT 56 is easily affected and deteriorated by a developer of a development process of the photolithography process for patterning as well as the strip liquid of a strip process of the photo-resist. Consequently, it is difficult to control the patterning method for forming the organic semiconductor layer 64 as a channel of the organic TFT 56. Accordingly, the organic semiconductor layer 64 may be very hard to apply to the TFT array substrate.